NEUTER INSECTS. 475 



passive, the work just quoted abundantly proves. Now when neuters 

 are produced from males their duties differ from the neuters produced 

 from females, and, as we have supposed that the duties now performed 

 by sterile females were once performed by fertile females, so we may 

 now conclude that the duties performed by sterile males were once 

 performed by the perfect males. The worth of this conclusion will be 

 presently seen. 



The remaining difficulty is to account for the fact that with many 

 insects the neuters differ considerably from the fertile insects. Thus, 

 for example, with the termites or white ants ; these are the perfect 

 males and females, the soldiers, which are aborted males, and the 

 workers, which are aborted females. The males and females have 

 wings, the neuters are wingless ; the workers undertake architectural 

 duties, act the part of nurses, etc., while th^ soldiers defend the nest 

 from attacks. Both workers and soldiers are blind, but whereas the 

 workers have a somewhat circular head and small jaws, the soldiers 

 have a comparatively enormous head, and strong resisting mandibles. 

 In what manner then, or through what cause, could the head of this 

 soldier termite differ so greatly from either that of the perfect male 

 or perfect female ? Or, since it is impossible in any given case to ex- 

 plain all the details satisfactorily, let it be asked how it is that so many 

 neuter insects differ from their parents. 



Neuters are either sterile males or sterile females, and in many 

 cases do not differ greatly from their fertile progenitors ; the social 

 bees and wasps are examples of this. On the theory advanced, it has 

 been supposed that originally the neuter differed only from the per- 

 fect insect in that it had a rudimentary reproductive system. Now, 

 suppose, to take an imaginary example, that in a colony of ants there 

 are only males and females ; that the duty of the male is the defense 

 of the nest against encroaching enemies, and that the duties of the 

 females are to build the nest, lay eggs, and take charge of the young. 

 Of course the males and females having different duties to perform 

 will have their structures differently modified ; say, in our case, the 

 male has a largely developed head like the soldier-termite, the female 

 a head like the worker-termite. Going a step further, neuters begin 

 to appear, the aborted male still performing soldier duty, the aborted 

 female still attending to its domestic duties. The neuters continuing 

 to increase as we know they have increased, and the true males and 

 females decreasing in number as we know they have decreased, a state 

 of affairs is reached in which it is essential to the welfare of the colony 

 that the male should confine himself to fertilizing the female, the fe- 

 male principally confine herself to laying eggs. 



Thus far, the fertile and infertile males, the fertile and infertile 

 females, have resembled each other ; but disuse of parts induces retro- 

 grade metamorphosis, or modification or suppression of useless parts. 

 If the males no longer use their heads and jaws to protect the com- 



